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(11) | EP 0 115 654 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
High-pressure sodium discharge lamp Hochdrucknatriumentladungslampe Lampe à décharge à sodium à haute pression |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[0001] The invention relates to a high-pressure sodium discharge lamp comprising a discharge envelope which is provided with a filling containing sodium and a rare gas and through the wall of which is passed at least one current-supply conductor to a solid non-sintered electrode arranged in the discharge envelope and consisting of tungsten and free of alkaline earth metals. Such lamps, which are known from GB-A-2 083 692, have the advantage of a high luminous efficacy. The discharge envelope then consists of a crystalline oxide resistant to sodium vapour, such as, for example, mono-crystalline sapphire or densely sintered polycrystalline aluminium oxide. The filling of the discharge envelope may contain besides sodium and one or more rare gasses also mercury.
[0002] It has been found that in the known lamp in many cases the electrode is attacked, which in the end causes the electrode to break off, sometimes after an operating time of approximately 1000 hours, which causes the actual life of these lamps to be seriously less than the envisaged life of at least 2000 hours.
[0003] The invention has for its object to provide means to suppress this life-limiting phenomenon. Therefore, according to the invention, in the lamp of the kind mentioned in the opening paragraph the electrode contains rhenium in a quantity of 1 to 27% by weight.
[0004] It has been found that rhenium in such a small quantity of 1% by weight effectively suppresses the attack of the electrode. With a content of 3% by weight, just like with a higher content of, for example, 27% by weight, attack is completely prevented from occurring. Since rhenium is very expensive, that content of rhenium will be chosen which under the given conditions suppresses the attack in such a manner that this attack does not lead to the end of the life of the lamp. Therefore, in the majority of cases, a rhenium content of 1 to 3% by weight will be chosen.
[0005] The attack ascertained takes place, viewed in the longitudinal direction of the electrode, very locally, but occurs all round the circumference. The attack has the form of a removal of material on the side facing the discharge immediately followed by a deposition of material on the side remote from the discharge. A further analysis has shown that the attack takes place at the area at which the electrode has a temperature lying between 2000 K and 2500 K. The mechanism on which the attack is based, however, has not been explained. This entails that also the effect of rhenium has remained unknown.
[0006] It is true, in literature (US-A-3 621 322) a sintered electrode containing tungsten and rhenium has been suggested for a high-pressure discharge lamp. The electrode then further contains tantalum carbide. However, experiments have shown that under conditions prevailing in high-pressure sodium discharge lamps tantalum carbide gives rise to a rapid blackening of the discharge envelope due to sputtering and evaporation. Moreover, a sintered electrode has the property that sputtering will occur more readily than in a non-sintered electrode.
[0007] In order to promote electron emission by the electrode so that a minimum electrode temperature will be sufficient during operation of the lamp, in the case of a lamp according to the invention, the material of the electrode may be provided with thorium oxide or yttrium oxide. Preferably, in lamps according to the invention, the electrode is pin-shaped and emitter-free, whilst the rare gas used is xenon, which at 300 K has a pressure of at least 13 kPa. High-pressure xenon is found to have the advantage that a blackening of the discharge envelope due to electrode material sputtered and evaporated during the starting stage is counteracted. Furthermore an emitter-free electrode is easier to manufacture whilst it has appeared that the emissive property of said electrode hardly fall behind an electrode provided with thorium oxide or yttrium oxide. Moreover, such an electrode is particularly suitable for use in small lamps, for example with a power consumption of 100 W or less.
[0008] An embodiment of a lamp according to the invention will be described more fully with reference to a drawing, in which:
Fig. 1 shows diagrammatically a lamp according to the invention, and
Fig. 2 shows in detail in sectional view the discharge envelope of this lamp.
[0009] The lamp shown in Fig. 1 has an outer bulb 1 provided with a lamp cap 2. The outer bulb encloses a discharge envelope 3 provided with two electrodes 4, 5. The electrode 4 is connected through a current-supply conductor 8 to a connection contact of the lamp cap 2. The electrode 5 is connected through a current-supply conductor in an analogous manner.
[0010] The discharge envelope 3 is composed, as shown in Fig. 2, of a discharge space enclosed by an elongate wall portion 3a, this wall portion being provided at its both ends with end portions 3b. The wall portion 3a and the end portions 3b consist of densely sintered aluminium oxide and are connected to each other, for example, by means of sintered connections 7. The outer diameter of the wall portion 3a is 3.5 mm. The discharge envelope is provided with two electrodes 4, 5, which have the form of pins of tungsten containing 3% by weight of rhenium and are secured on pin-shaped current-supply members 40, 50 of Nb. The pin-shaped tungsten electrodes of the lamp described have a diameter of 0.3 mm. The electrode gap is 13 mm. The pin-shaped current-supply members 40, 50 are connected in a gas-tight manner to the end portions 3b by means of a sealing glass. The filling of the discharge envelope of the lamp described contains xenon at a pressure of 50 kPa at 300 K and 5 mg amalgam consisting of 27% by weight of Na and 73% by weight of Hg. The lamp is operated through an inductive stabilization load of 390 0 at a supply source of 220 V, 50 Hz. For starting purposes, the lamp is connected parallel to a starter. The lamp may then be provided with an external auxiliary electrode. The power consumed by the lamp is approximately 30 W, the lamp current being 0.47 A. The luminous efficacy is approximately 441 m/W at colour temperature of the emitted radiation of 2450 K. In the operating condition of the lamp, the electrode tips of the electrodes 4, 5 assume a temperature of approximately 2700 K. The lamp described is particularly suitable for interior illumination purposes.
1. A high-pressure sodium discharge lamp having a discharge envelope which is provided
with a filling containing sodium and a rare gas and through the wall of which at least
one current-supply conductor is passed to a solid non-sintered electrode arranged
in the discharge envelope, wherein said electrode is free of alkaline earth metals
and consists of tungsten and rhenium, the amount of rhenium being of 1 to 27% by weight.
2. A lamp as claimed in Claim 1, characterized in that the rare gas is xenon, which
at 300°K has a pressure of at least 13 kPa (100 Torr), and in that the electrode is
pin-shaped and in that it is emitter-free.
1. Hochdrucknatriumentladungslampe mit einem Entladungsgefäss, das mit Natrium und
einem Edelgas gefüllt ist und durch seine Wand zumindest einen Stromzuführungsleiter
nach einer massiven nicht gesintertem Elektrode innerhalb des Entladungsgefässes geführt
ist, wobei die Elektrode frei von Erdalkalimetallen ist und aus Wolfram und Rhenium
besteht, wobei die Rheniummenge 1 bis 27 Gew.% beträgt.
2. Lampe nach Anspruch 1, dadurch gekennzeichnet, dass das Edelgas Xenon ist, das
bei 300°K einen Druck von zumindest 13 kPa (100 Torr) hat, und dass die Elektrode
stiftförmig und emitterfrei ist.
1. Lampé à décharge dans le sodium à haute pression présentant une enceinte à décharge
qui est munie d'un remplissage contenant du sodium et un gaz rare et dont la paroi
est traversée par au moins une entrée de courant s'étendant vers une électrode non
frittée disposée dans l'enceinte à décharge, ladite électrode étant exempte de métaux
alcalino-terreux et est constituée par du tungstène et du rhénium, la quantité de
rhénium étant de 1 à 27% en poids.
2. Lampe selon la revendication 1, caractérisée en ce que le gaz rare est du xénon
qui, à 300 K présente une pression d'au moins 13 kPa (100 torrs) et que l'électrode
est en forme de broche et est exempte d'émetteur.